In many signal processing applications, signals must be offered in various situations alternative routes through active components. An exemplary case is provided by a radio transmitter's power amplifier arranged so as to operate at different power levels according to need. It is known that the transmission power of a mobile phone, for example, varies in accordance with the magnitude of the power needed to establish a functional and error-free uplink connection between the mobile phone and a base station. The efficiency of individual radio-frequency amplifiers varies as a function of the output power so that the amplifier is usually the most efficient at high output power values. When such a radio-frequency amplifier operates at a lower power, the efficiency is poor and electric power is wasted.
To save the batteries of portable radio apparatuses the consumption of electricity in all parts of the apparatuses should be minimal in all situations, wherefore different solutions have been developed to improve the efficiency of the amplifiers at low power levels. A known solution is to place in the power amplifier of a transmitter, instead of a single radio-frequency amplifier, two or more parallel amplifiers optimised to different power levels. The outputs of the parallel amplifiers are connected to a selection switch to select the amplifier which is closest to optimum for the current power level. In the GSM (Global System for Mobile Telecommunications), for instance, the selection is carried out as follows: the base stations measure the strengths of the signals received from mobile terminals and send out commands lo the terminals, instructing the terminals to adjust the transmission power. When a received signal becomes weaker, the base station instructs the terminal to increase the transmission power or hand over the connection to another base station. If a received signal is so strong that the transmission power of the terminal can be decreased without compromising signal quality, the base station instructs the terminal to decrease the transmission power. In the GSM system, the transmission power of a hand phone may typically vary from 1 mW to 2 W (in the dBm scale, from 0 dBm to +33 dBm).
It can be considered a disadvantage of prior-art solutions that a selection switch connected in series with parallel amplifiers has to be capable of withstanding the maximum output power of the amplifier with the highest power so that the switch easily becomes rather large in size and expensive to manufacture. In addition, a selection switch connected in series on the path of the signal causes losses, which is in contradiction with the power-saving goals of the arrangement. Switch arrangements cause unnecessary losses also in other cases where the signal has to be directed to pass through at least two mutually alternative active components. An, example of such a switch arrangement is an antenna switch of a radio apparatus based on time division duplex, TDD, which couples to the antenna of the radic apparatus either the power amplifier output of the transmitter or the low-noise preamplifier input of the receiver.